Herd Instinct

Also from Answers.com... Cancer Treatments Learn about current and promising cancer treatments.

(psychology) Psychic need for identification with a group.

A mentality characterized by a lack of individuality, causing people to think and act like the general population.

Investopedia Says:
This term is used in the investing world to refer to the forces that cause unsubstantiated rallies or sell-offs.

Related Links:
Investors of course are human, but find out here how our bad habits can cause market turbulence. How Investors Often Cause The Market's Problems
Use these rules to guide you on the road to financial freedom. The 10 Commandments Of Investing
Curious about how emotions and biases affect the market? Find some useful insight here. Taking A Chance On Behavioral Finance